In order to achieve higher data rate transmissions, high speed serial interfaces are widely used recently in electrical products, such as handheld devices, wireless cloud applications, personal computers, and others. In operations at high frequencies such as 3 GHz or above, electrical currents and voltages present in the circuit often suffer from electromagnetic interference (EMI). A spread spectrum clock generator (SSCG) is an effective way of using frequency modulation to reduce EMI. The fractional-N type PLL with a sigma delta modulator (SDM) is a type of SSCG that achieves high resolution with high operation frequency. The fractional-N PLL with the SDM, however, generates SDM quantization noise that deteriorates jitter performance.